1. Field of the Invention
The invention is directed to methods of magnetic resonance analysis and, in particular, to such methods for magnetic resonance imaging and spectroscopic analysis of intra thoracic anatomic structures, such as the aorta, from the esophagus of a patient. The invention is also related to a magnetic resonance analysis apparatus.
2. Description of the Prior Art
Current standard techniques for imaging the thoracic aorta include X-ray computed tomography (CT), standard magnetic resonance imaging (MRI) (e.g., body-coil MRI), transesophageal echocardiography (TEE), and contrast aortography. Each of these techniques suffers some important limitation in its ability to allow detailed mapping of the aortic wall and its anatomic and functional lesions.
Standard MRI and CT lack adequate resolution of the aortic wall for precise characterization of aortic atheromata in vivo, and are not able to provide measurements of focal variations in vessel wall compliance or distensibility (e.g., aortic wall tissue tagging information).
TEE allows real time imaging, but suffers from both an inability to image clearly that portion of the aortic wall which is directly against the esophagus due to the near field effect of ultrasound (e.g., portions of the thoracic aortic wall, particularly in the arch), and from an inability to register images to a fixed frame of reference, making precise mapping of aortic lesions problematic. Kasprzak, J. D., et al., Three-dimensional echocardiography of the thoracic aorta, Eur. Heart. J., vol. 17, pp. 1584-92, 1996, discloses an attempt to circumvent this limitation using a technique to control movements of the probe while imaging in multiple planes with subsequent off-line 3-D image reconstruction. It is believed that the system is relatively cumbersome and not fully successful in obtaining xe2x80x9cadequatexe2x80x9d images in a select group of 21 patients.
Montgomery, D. H., et al., Natural history of severe atheromatous disease of the thoracic aorta: a transesophageal echocardiographic study, J. Am. Coll. Cardiol., vol. 27, pp. 95-101, 1996, discloses an example of a sermi-quantitative atherosclerosis grading scheme which depends upon orthogonal views to estimate the three-dimensional characteristics of aortic lesions, but does not circumvent the inherent advantage of MR over ultrasound imaging at defining atheroma structure. See, for example, Martin, A. J., et al., Arterial imaging: comparison of high-resolution US and MR imaging with histologic correlation, Radiographics, vol. 17, pp. 189-202, 1997.
Contrast aortography, which is often considered to provide one of the best standards for aortic imaging, is actually a misnomer since none of the tissues which make up the aortic wall are visualized directly. Instead, only lesions which protrude into the lumen and focally displace the contrast agent can be xe2x80x9cseenxe2x80x9d as an absence of signal. Any inferences about the vessel wall depend upon a comparison of contrast displacement from the area of the lesion to the displacement around an adjacent xe2x80x9creferencexe2x80x9d segment of normal artery, which is often unavailable. See, for example, Thomas, A. C., et al., Potential errors in the estimation of coronary arterial stenosis from clinical arteriography with reference to the shape of the coronary arterial lumen, Br. Heart J, vol 55, pp. 144-150, 1993. It is believed that any statements about the thickness and stiffness of the vessel wall at the site of a contrast filling defect are purely conjectural.
For these reasons, some investigators prefer the term lumenography to describe standard contrast angiography in general (of which contrast aortography is a specific example). Libby, P., Lesion versus lumen, Nature Medicine, vol. 1, pp., 17, 18, 1995.
MRI has a distinct advantage over TEE in that tissue characterization is possible. See, for example, Toussaint, J. F., et al., Magnetic resonance images lipid, fibrous, calcified, hemorrhagic, and thrombotic components of human atherosclerosis in vivo, Circulation, vol. 94, pp. 932-38, 1996; and Correia, L. C. L., et al. By performing MRI using an intravascular receiver, higher resolution imaging can be achieved at the cost of invasiveness. See, for example, Ocali, O., et al.; Martin, A. J., et al., J Magn Reson Imaging, vol. 8, pp. 226-34; Martin, A. J., et al., Radiographics, vol. 17, pp. 189-202; and Atalar, E., et al., Magn Reson Med, vol. 36, pp. 596-605.
Intravascular MR has overcome many of the limitations of CT and standard MRI at the cost of invasiveness. Martin, A. J., et al., High-resolution MR imaging of human arteries, J. Magn. Resort. Imaging, vol. 5, pp. 93-100, 1995, discloses an intra-aortic catheter coil which is employed to image the aortic wall in a pig model, although the coil is relatively large and requires ligation of the aorta.
Atalar, E., et al., High resolution intravascular MRI and MRS using a catheter receiver coil, Magn. Reson. Med., vol. 36,pp. 596-605, 1996, discloses a 9 French (i.e., 3 mm outer diameter) catheter coil designed specifically for intravascular imaging. This validates the ability to quantitate atherosclerotic plaque burden and intraplaque composition against histopathology in cadaveric human aortae.
Although intravascular MRI is emerging as a valuable tool for studying aortic disease, in vivo human studies must await proper safety testing and regulatory approval.
There has been considerable interest on factors influencing atherosclerotic plaque stability. Plaque composition may predict plaque stability, and interventions that alter plaque composition may change the likelihood of plaque rupture and clinical events. Ferrari, E., et al., Atherosclerosis of the thoracic aorta and aortic debris as a marker of poor prognosis: benefit of oral anticoagulants, J Am Coll Cardiol.,vol. 33,pp. 1317-22, 1999, discloses that these hypotheses are supported by indirect evidence, although direct testing in vivo has not been possible.
The thoracic aorta represents a valuable window for the study of atherosclerotic plaque burden and vulnerability. See, for example, Fazio, G. P., et al.; Amarenco, P., et al., Atherosclerotic disease of the aortic arch and the risk of ischemic stroke, N Engl J Med., vol 331,pp. 1474-79, 1994; Cohen, A., et al., Aortic plaque morphology and vascular events: a follow-up study in patients with ischemic stroke. FAPS Investigators. French Study of Aortic Plaques in Stroke, Circulation, vol. 96, pp. 3838-41, 1997; and Witteman, J. C., et al., Aortic calcified plaques and cardiovascular disease (the Framingham Study), Am J Cardiol, vol. 66, pp. 1060-64, 1990.
The prior art also shows that atherosclerotic disease of the thoracic aorta predicts cerebrovascular events, coronary disease/events, and death.
Without invading a vascular space, it is known to obtain similar information by receiving the signal from an adjacent body structure. The concept of placing a radio frequency (RF) receiver coil into a body cavity in order to image an adjacent structure by MR is disclosed by Narayan, P., et al., Transrectal probe for 1H and 31P MR spectroscopy of the prostate gland, Magn. Reson. Med., vol. 11,pp. 209-20, 1989 (an endorectal RF receiver coil is employed to image the canine prostate); and by Schnall, M. D., et al., Prostate: MR imaging with an endorectal surface coil, Radiology, vol. 172, pp. 570-74, 1989 (an expandable endorectal RF receiver coil is employed to image the prostate in 15 humans having biopsy proven prostate carcinoma and two normal volunteers).
U.S. Pat. No. 5,348,010 discloses a rectal MRI receiving probe for use in imaging the prostate.
It is known to employ an endovaginal coil to image the vagina and adjacent structures. See, for example, Siegelman, E. S., et al., High-resolution MR imaging of the vagina, Radiographics, vol. 17,pp. 1183-1203, 1997.
U.S. Pat. No. 5,355,087 discloses the use of a probe in MRI or spectroscopy related to either the prostate or cervix. An RF receiving coil is inserted into the rectum or vagina in effecting these respective measurements.
It is also known to study the aorta by employing an expandable coil-type RF receiver in the inferior vena cava. See Martin, A. J., et al., An expandable intravenous RF coil for arterial wall imaging, J Magn. Reson. Imaging, vol. 8, pp. 226-34, 1998. While this approach avoids the need to invade the aorta, it necessitates placement of a large caliber central venous catheter, with associated risks.
U.S. Pat. No. 5,928,145 discloses magnetic resonance imaging (MRI) and spectroscopic analysis of small blood vessels using a flexible probe of relatively small dimension. A loopless antenna is employed wherein a coaxial cable is structured to be received within the intravascular system, a blood vessel such as a human vein, the femoral artery of a live rabbit for imaging the aorta thereof, a naturally occurring passageway in a human being, an opening of the pancreatic duct, or a tortuous passageway of a patient. In one embodiment, the optimal length of the antenna is about 7 cm to 10 cm and the loopless antenna has a maximum width of about 0.5 mm to 1.0 cm. Matching and decoupling circuits are employed. Preferably, the loopless antenna is flexible for purpose of movement in a tortuous path. U.S. Pat. No. 5,928,145 does not disclose any esophageal insertion of an antenna nor any insertion of an antenna in one body passageway to image body portions external to that passageway.
U.S. patent application Ser. No. 08/979,121 discloses the use of a body coil and support member and a catheter antenna employed for insertion into the body. An endoscope is inserted through the patient""s mouth into the esophagus with an antenna in the form of a coaxial cable being delivered therethrough. The antenna is delivered to the esophagus by the endoscope which serves as a support surface therefor. Cylindrically encoded images are produced around the endoscope.
It is believed that an endoscope generally requires the sedation of the patient.
U.S. Pat. No. 5,699,801 discloses a flexible receiver coil for introduction into small blood vessels for purposes of accessing atherosclerotic areas. The receiver coil is introduced into or adjacent to the specimen, such as a patient. The coil is inserted within a catheter, an endoscope, a biopsy needle, or other probe-type medical devices.
U.S. Pat. No. 5,792,055 discloses the use of a coaxial cable functioning as an antenna in MRI procedures with particular emphasis on vascular uses.
U.S. Pat. No. 5,432,450 is directed toward an MRI probe having internal and external conductors.
U.S. Pat. No. 5,419,325 is directed to MRI and spectroscopy and discloses the use of a Faraday catheter inserted into a blood vessel of a patient.
U.S. Pat. No. 5,417,713 is directed toward a defribillating system for the heart which is inserted into the esophagus.
U.S. Pat. No. 5,211,166 discloses a biopsy needle or similar instrument or radiation-containing capsule, which is adapted to be detected by MRI procedures.
U.S. Pat. No. 4,572,198 discloses an MRI catheter which facilitates location of the catheter tip.
U.S. Pat. No. 5,170,789 discloses an insertable probe which has a two-component structure (i.e., a handle portion and an insertable portion having a coil). MRI and spectroscopy is employed to study deeply located organs, such as the rectum, colon, prostate, bladder, cervix and other tissue in close proximity to these or other internal organs.
The prior art shows that there is room for improvement in the known methods and apparatus for magnetic resonance imaging and spectroscopic analysis of the aorta and other intra thoracic anatomic structures.
As one aspect of the invention, a method of transesophageal magnetic resonance analysis comprises providing a loopless antenna; receiving a portion of the loopless antenna in a gastric tube; inserting the gastric tube which receives the loopless antenna in the esophagus of a patient; employing a matching and tuning circuit for the loopless antenna external to the patient; electrically connecting the matching and tuning circuit to a magnetic resonance scanner; and employing the magnetic resonance scanner for displaying an image of the aorta of the patient.
The gastric tube may be a Levin gastric tube. Preferably, the gastric tube is employed as a nasogastric tube, and transnasal placement of the nasogastric tube is employed in the esophagus of the patient.
As another refinement, the loopless antenna may be employed to confirm proper placement of the gastric tube in the esophagus of the patient.
As another aspect of the invention, a transesophageal magnetic resonance analysis apparatus comprises a loopless antenna having a proximal portion and a distal portion; a gastric tube for receiving the distal portion of the loopless antenna and for inserting the distal portion of the loopless antenna in the esophagus of the patient; a matching and tuning circuit having a first port and a second port which is electrically connected to the proximal portion of the loopless antenna; magnetic resonance scanner means for displaying an image of the aorta of the patient; and a cable electrically connecting the first port of the matching and tuning circuit to the magnetic resonance scanner means.
As a further aspect of the invention, a method of transesophageal magnetic resonance analysis of a patient comprises providing an antenna; receiving the antenna in a gastric tube; inserting the gastric tube which receives the antenna in the esophagus of the patient; employing a matching and tuning circuit for the antenna external to the patient; electrically connecting the matching and tuning circuit to a magnetic resonance scanner; and employing the magnetic resonance scanner for magnetic resonance imaging or spectroscopic analysis of an intra thoracic anatomic structure of the patient.
As another aspect of the invention, a transesophageal magnetic resonance analysis apparatus for a patient comprises an antenna; a gastric tube for receiving the antenna and for inserting the antenna in the esophagus of the patient; a matching and tuning circuit having a first port and a second port which is electrically connected to the antenna; magnetic resonance scanner means for magnetic resonance imaging or spectroscopic analysis of an intra thoracic anatomic structure of the patient; and a cable electrically connecting the first port of the matching and tuning circuit to the magnetic resonance scanner means.
These and other objects of the present invention will be more fully understood from the following description of the invention with reference to the illustration appended hereto.